The present disclosure relates to a method of lining a process tank, and in particular, the present disclosure relates to lining a process tank using sheets that are extrusion welded together by infusing molten thermoplastic material within the interfaces of adjacent sheets. The present disclosure also relates to a resultant lining formed by the welded sheets and infused material. For descriptive purposes, the term “liner” and “lining” are used interchangeably.
Currently, contents such as acids and chemicals are stored in tanks usually in the form of process tanks. These tanks relate to immobile types that may be installed above or below the ground, but also for the transportable types that are part of the over-the-road semi-trailers. The tanks may also be used on or in marine vessels as well as railroad cars. The size of the tank is not material, but the large tanks typically hold 1,000 gallons or more. Moreover, process tanks are particularly adaptable for tanks intended for highly corrosive liquids, but also may be used in conjunction with other pourable materials such as grain and pellets.
Most process tanks of the type considered are steel tanks which, over a period of time, may become corroded as a result of the fluids stored therein, or because of the rusting action of the exterior elements (ground water, rain, etc.). If the material stored in such tanks is corrosive, the corrosive material can contact the tank. In this situation, the life expectancy of the tank is relatively short and thus it becomes not only extremely expensive for replacement, but also highly dangerous for people and the environment. Furthermore, there is danger in the event that the tanks leak or are ruptured, or somehow fail to retain the contents and leak the contents into the ground (if the tanks are subterranean). If they are above-the-ground storage tanks, or to the passing public if the tanks are over-the-road type, there is danger along the highways and to the passing public. Accordingly, many process tanks utilize a protective liner or lining.
One common type of liner is a pre-fabricated “drop-in” liner. While drop-in liners may be machine welded (radio frequency welding is commonly used for these liners), the drop-in liners have disadvantages with respect to a bonded lining. During the drop-in process, air is entrapped behind the liner, which can condense and cause the mild steel tank to rust. Furthermore, during the drop-in process, creases form in the liner sheet, which stresses the liner material and leads to premature cracking and failure. Additionally, a tank part may catch the crease or protruding wrinkle and cause tear damage to the drop-in liner. When the drop-in liner develops a leak, solution seeps behind the liner pushing it off the walls or bottom and causing the liner itself to move into the process tank area resulting in operational problems. Once solution is behind a drop-in liner, the liner is very difficult to repair, since it may be almost impossible to find the source of the leak. Replacing the drop-in liner creates significant downtime, especially for electroplating tanks with auxiliary equipment affixed to the tank rim, i.e., ventilation hoods, piping, anode and cathode bars, heat exchangers and probes, level control devices, etc
Another common liner is a bonded to metal lining. As will be discussed, this liner uses manual “flat strip” welds on the butted side panels and “corner strip” welds on the vertical joining walls and side to bottom joints.
In current lining procedures, installation personnel prepare the interior of the surface of the tank 10 (FIG. 1) to receive the liner 14. This preparation includes surface blasting the interior of the tank 10 and subsequent cleaning of the interior of the tank 10. With respect to the liner 14, the installer cuts sheets of liner 16 (FIG. 2) from a roll of liner material. At the installation site, the installer applies an adhesive to the now cut sheets of liner 16. Then, the installer manually applies the liner sheets 16 to the interior of the tank 10. As known in the art, heat may be applied to the liner sheets 16 to assist in applying the liner sheets 16 to the tank wall. Tanks typically have protrusions such as tank welds that bond the tank walls to the tank bottom. These tank welds protrude into the interior of the tank. Even careful placement of the sheets 16 will result in gaps between the sheets 16 that are placed over the protruding welds. In other words, the sheets 16 will lay over the protrusions further enhancing the gaps between the sheets 16.
As shown in FIG. 2, current cutting procedures result in uneven and/or rough edges 18 for each liner sheet 16. When the installer adheres the liner sheets 16 to the tank 10 and next to each other, the rough edges 18 of the liner sheets 16 do not evenly match resulting in gaps 20 forming between the sheets 16. When the installer cuts relatively smooth edges 18, installation gaps 20 still exist between the adjacent liner sheets 16 due to the difficult and labor intensive installation process (FIG. 3). For example, the liner sheets are heavy and difficult to manage as the installer handles the sheets while positioned within the tight constraints of the process tank, i.e., a confined space with elevated temperatures. As such, the installer may apply adjacent sheets 16 in a non-uniform layout and/or with a distance between them, further enhancing the gaps 20 between the edges 18 of the sheets 16. Applying the sheets at a corner of the tank 10 is particularly troublesome due to the space and angle considerations of the corner of the tank 10.
After applying the liner sheets 16, the installer welds a weld strip 22 (known as a “cap over flat strip weld” or a “cap over corner strip weld”) along the interface between a pair of adjacent sheets 16 (FIGS. 2 and 3). The installer manually welds the weld strip 22 to the adjacent liner sheets 16. The welders used in this process heat the weld strip 22 to the liner sheets 16. Similar to the application of the liner sheets 16, hand welding the weld strips 22 is a labor-intensive process. Maintaining consistent pressure with the welder is difficult since the touch of the installer applies the pressure. Additionally, it is difficult with the hand welder to maintain a constant distance between the welding nozzle and the welding strip. Furthermore, the weld strip may melt faster than the liner sheet, so the welding process must be done with special care. The sheets must be heated to a glossy state, yet the weld strip or the sheets cannot be charred, as that would result in a failed weld.
The installer typically welds from the top of the liner sheet 16 to the bottom. As the process tank may have a height such as twelve feet, this height causes starts and stops as opposed to continuous welds with tightly controlled temperatures and consistency in both pressure and timing. In addition, welding occurs within the tight constraints of the process tank such that the installer does not provide a constant weld over any length of time. The tedious and laborious process for strip welding not only applies to welding strips to corner sheets; but also, applies to welding strips for sheets applied to the walls of the process tank 10.
The human element of welding the strips 22 leads to weak welds (i.e., inconsistency of temperature, pressure and timing—the critical variables for welds) and leads to voids or “pinholes” 24 within the weld that bonds the weld strip 22 to the liner sheets 16 (FIG. 4). The pinholes 24 shown in FIG. 4 are exaggerated for purposes of clarity. Although the welded strip 22 may pass a “spark test” commonly used in the art, these pinholes 24 lead to problems for the process tank 10 as will be discussed. Furthermore, the corner weld that bonds sides and the bottom of the process tank further exaggerates the effects of the gaps 20 and the pinholes 24 since the liner sheet 16 must position over the corner weld of the process tank 10. This corner weld or other obstacle leaves a void between the sheet 16 and the tank weld.
When the tank 10 is filled with fluid 12 (FIG. 1) such as an acid, the pressure of the fluid forces the fluid through the pinholes 24. Consequently, the fluid forces through the gaps 20 and disperses between the liner 14 and the tank 10. This leaked fluid then corrosively attacks the tank wall. Additionally, this leaked fluid may also corrosively attack the adhesive interface between the liner 14 and the tank wall resulting in the liner 14 pulling away from the tank wall. Accordingly, the gaps 20 and the pinholes 24 between the liner sheets 16 lead to adverse and dangerous conditions. When the installer repairs the welded strip, the heat from the repair welder draws the leaked fluid toward the interface of the adjacent liner sheets, wherein this fluid further attacks the tank wall positioned behind the repaired weld strip.
Process tanks with bonded linings require a lining that eliminates gaps between edges of adjacent liner sheets. Process tanks also require machine quality consistent strong welds created with consistent pressure, temperature and timing that effectively seals the tank from the contents that contact the liner.
The foregoing and other objects, features, and advantages of the disclosure as well as presently preferred embodiments thereof will become more apparent from the reading of the following description in connection with the accompanying drawings.